Project 1 will investigate the regulation of inflammation and lipid homeostasis in the context of macrophage foam cell formation and the pathogenesis of atherosclerosis. These studies will leverage our recent discovery that desmosterol, an intermediate in the cholesterol biosynthetic pathway, unexpectedly plays a key role in integration of the homeostatic response to excess cholesterol by activating LXR target genes required for cholesterol efflux, suppressing SREBP target genes involved in cholesterol and fatty acid biosynthesis, and inhibiting inflammatory responses by LXR-dependent and independent mechanisms. These findings have a number of unanticipated implications for understanding the pathophysiology of atherosclerosis and suggest new therapeutic approaches. In Specific Aim 1, we will test the hypothesis that de-compensation of cholesterol homeostasis and 'expression of genes that amplify inflammatory responses in macrophage foam cells results from extrinsic, pro-inflammatory signals that are generated in the artery wall and induce TLR signaling. In Specific Aim 2, we will test the hypothesis that in addition to serving as TLR-induced transcriptional activators of inflammatory-response genes, NFKB and IRF3 mediate TLR dependent suppression of LXRs. Delineation of the mechanisms by which NFKB and IRF3 inhibit the lipid homeostatic and anti-inflammatory activities of LXRs may suggest new approaches for therapeutic intervention. In Specific Aim 3, we will test the hypothesis that increasing endogenous levels of desmosterol or mimicking its activities will promote sterol excretion and inhibit the development of atherosclerosis. Conventional LXR ligands promote cholesterol efflux and inhibit atherosclerosis in mice, but are not useful drugs because they induce the expression of SREBP1c and cause marked hypertriglyceridemia. In contrast, desmosterol activates LXRs but suppresses SREBP processing. In this aim, we will investigate whether treatment with a putative desmosterol mimetic or raising endogenous desmosterol levels can be used as approaches to restore cholesterol homeostasis in lesion macrophages without causing adverse effects on circulating lipoprotein levels.